1. Field of the Invention
This invention relates to a method for the production of a semiconductor laser device which emits laser light from an end facet thereof, and more particularly, it relates to an improved method for the production of such a semiconductor laser device which can attain high reliability even when operated at a high output power level for a long period of time.
2. Description of the Prior Art
A semiconductor laser device which emits laser light from an end facet thereof is a typical example of the semiconductor devices produced by use of the cleavage of semiconductor crystals. A semiconductor laser device of this type has a Fabry-Perot resonator having a pair of semiconductor facets and functioning on the basis of the difference in refractive index between the semiconductor crystals and the air outside the device.
In recent years, semiconductor laser devices such as described above have widely come into practical use as light sources for optical disc driving units and the like. When semiconductor laser devices are used as the light sources for write-once optical disc driving units or rewritable optical disc driving units, they are required to have high reliability even at a high output power level of about 40 to 50 mW. Furthermore, for the purpose of attaining higher operational speed of an entire system including an optical disc driving unit, there is a demand for semicopnductor laser devices which can attain laser oscillation at a still higher output power level. When semiconductor laser devices are used as the light sources for high-resolution laser printers or for optical pumping of solid state laser devices such as a YAG laser, they are required to attain laser oscillation at an output power level of 100 mW or more.
The high output power operation of such a semiconductor laser device, however, causes the deterioration of its end facet from which laser light is emitted. The deterioration in the light-emitting facet increases the current required for driving the semiconductor laser device, and eventually it becomes impossible for the laser device to attain laser oscillation. Therefore, with respect to semiconductor laser devices, it is difficult to attain high reliability at a high output power level.
The principal cause for the deterioration of the light-emitting facet is now described. First, heat is generated locally at the light-emitting facet due to the high optical density at this facet and also due to non-radiative recombination caused by the surface state. As the temperature in the area near the facet increases, the band gap in that area becomes smaller, which in turn increases the absorption of light. The increase in the light absorption generates carriers, which are then trapped in the surface state, and nonradiative recombination of the carriers occurs. This further generates heat in the area near the light-emitting facet. This process is repeated until the temperature in the area near the facet reaches the melting point of the semiconductor, resulting in facet breakdown.
For the prevention of such deterioration in the resonator facet, there has been proposed a semiconductor laser device having a semiconductor layer with a band gap larger than that of an active layer (i.e., a large-band-gap layer) formed on the facet.
In the production of such a semiconductor laser device, resonator facets are first formed on a single wafer, and then large-band-gap layers such as mentioned above are formed on the facets.
For example, in a conventional method for producing this type of semiconductor laser device, resonator facets are formed on a wafer by dry etching, after which semiconductor layers having a band gap larger than that of an active layer are formed on the resonator facets. This process will be described in detail below.
FIGS. 2a to 2c are sectional views of a wafer taken along the direction of an optical waveguide, in the steps of the conventional method for producing a semiconductor layer device with a large-band-gap layer.
First, as shown in FIG. 2a, on an n-GaAs substrate 101, an n-A1.sub.0.33 Ga.sub.O.67 As first cladding layer 102, an n-A1.sub.0.08 Ga.sub.0.92 As active layer 103, a p-A1.sub.0.33 Ga.sub.0.67 As second cladding layer 104, and a GaAs contact layer 105 are successively grown by epitaxy, resulting in a buried BCM wafer.
Next, on the entire surface of the GaAs contact layer 105, a layered structure composed of two photoresists and a Ti film interposed therebetween is deposited, and then formed into stripes perpendicular to the direction of an optical waveguide, resulting in a multi-layer resist 106 with a prescribed striped pattern. Using the multi-layer resist 106 as a mask, the wafer is subjected to reactive ion beam etching. As a result, striped channels 120 perpendicular to the optical waveguide are formed in the wafer. The side walls of the striped channels 120 become resonator facets 107, as shown in FIG. 2b.
The above-described process of forming resonator facets by the use of dry etching is reported by Uchida et al., in "Preprints for the 32nd Annual Meeting of the Society of Applied Physics, Spring 1985 (1 P-ZB-8)".
After the formation of the resonator facets, the wafer is taken out into the ambient air, followed by the removal of the multi-layer resist 106. Then, an AlGaAs layer 108 which is a semiconductor crystal layer with a band gap larger than that of the n-A1.sub.0.08 Ga.sub.0.92 As active layer 103 is formed on each resonator facet 107 by vapor deposition such as metal organic chemical vapor deposition or molecular beam epitaxy, as shown in FIG. 2c.
Then, a p-sided electrode 109 and an n-sided electrode 110 are formed on the upper face of the GaAs contact layer 105 and on the bottom face of the n-GaAs substrate 101, respectively, as shown in FIG. 2c. Finally, the wafer is cleaved along each striped channel 120, and divided into a plurality of semiconductor laser devices. In this manner, a semiconductor laser device with large-band-gap layers on the resonator facets thereof is produced.
Another conventional method for producing this type of semiconductor laser device has been proposed which employs chemical etching to form resonator facets on a wafer. In this method, chemical etching is first performed to form resonator facets on a wafer, and then semiconductor layers with a band gap larger than that of an active layer are formed on the facets. This conventional process will be described in detail below.
FIGS. 3a to 3c are sectional views of a wafer taken along the direction of an optical waveguide, in the steps of the conventional process for producing a semiconductor laser device with a large-band-gap layer.
First, as shown in FIG. 3a, on a p-GaAs substrate 201, an n-GaAs current confining layer 202, a p-A1.sub.x Ga.sub.1-x As first cladding layer 203, a p-A1.sub.0.08 Ga.sub.0.92 As active layer 204, an n-A1.sub.x Ga.sub.1-x As second cladding layer 205, an n-GaAs contact layer 206 and an n-A1.sub.z Ga1.sub.-z As layer 207 are successively grown by epitaxy, resulting in an inner striped BTRS wafer. The Al mole fraction z of the A1.sub.z Ga.sub.1-z As layer 207 is obtained by adding 0.1 to the Al mole fraction x of the A1.sub.x Ga.sub.1-x As first and second cladding layers 203 and 205.
Then, on the entire surface of the A1.sub.z Ga.sub.1-z As layer 207, a layered structure composed of two photoresists and a Ti film interposed therebetween is deposited, and then formed into stripes perpendicular to the direction of an optical waveguide, resulting in a multi-layer resist 208 with a prescribed striped pattern. Using the multi-layer resist 208 as a mask, the wafer is subjected to chemical etching by the use of a solution containing H.sub.2 SO.sub.4, H.sub.2 O.sub.2 and H.sub.2 O in a volume ratio of 1:8:1. As a result, striped channels 220 perpendicular to the optical waveguide are formed in the wafer. The side walls of the striped channels 220 become resonator facets 209, as shown in FIG. 3b.
The above-described process of forming resonator facets on a wafer by chemical etching is reported by Wada et al., in "Preprints for the 45th Annual Meeting of the Society of Applied Physics, Autumn 1984 (13P-R-5)".
After the formation of the resonator facets, in the same manner as in the conventional process described with reference to FIGS. 2a to 2c, the wafer is taken out into the air, followed by the removal of the multi-layer resist 208. Then, an AlGaAs layer 210 which is a semiconductor crystal layer with a band gap larger than that of the p-A1.sub.0.08 Ga.sub.0.92 As active layer 204 is formed on each resonator facet 209, as shown in FIG. 3c.
Thereafter, an n-sided electrode 212 and a p-sided electrode 211 are formed on the upper face of the A1.sub.z Ga.sub.1-z As layer 207 and on the bottom face of the p-GaAs substrate 201, respectively, also as shown in FIG. 3c. Finally, the wafer is cleaved along each striped channel and divided into a plurality of semiconductor laser devices. In this manner, a semiconductor laser device with large-band-gap layers on the resonator facets thereof is produced.
In the above-described conventional processes, the wafer on which resonator facets have been formed is taken out into the ambient air before a large-band-gap layer is formed on each facet. Thus, the resonator facets are oxidized while being exposed to the air. Large-band-gap layers are formed on the thus oxidized resonator facets. Because of the oxidized portions of the resonator facets, the semiconductor laser device produced by the conventional processes has a surface state at the interface between each resonator facet and the large-band-gap layer formed thereon, thereby causing deterioration in the resonator facet.